1. Field of the Invention
The present invention relates to a pressure-resistant helically corrugated pipe intended mainly for underground use. For example, telecommunication cables, such as an electric cable and a telephone cable, are installed in such a corrugated pipe for the purpose of protecting such cables. Also, the corrugated pipe is used as a water main pipe, a sewer pipe, or a liquid transport pipe used in a plant or factory.
2. Background
Among conventional pressure-resistant helically corrugated pipes, the type of corrugated pipes adapted for underground use have been required to be highly pressure-resistant in order to withstand a great ground pressure. For this reason, there have already been manufactured and used underground pressure-resistant pipes of the type in which the pipe wall of a synthetic resin has an increased thickness so as to increase its pressure-resistant ability.
In such a synthetic resin pipe having an increased wall thickness to enhance the pressure resistance, however, a larger amount of synthetic resin is needed because of its increased wall thickness. This not only increases the cost of the material but also increases the weight of the pipe. As a result, the transport and underground installation of the pipe cannot be carried out easily. Further, with respect to the flexibility, although such a pipe is flexible to a certain extent, the radius of curvature of the curved pipe is inevitably large. Thus, the pipe cannot be bent to a small radius of curvature. When it is intended to enhance the flexibility of such a pipe, there has been encountered a problem that the pipe is inferior in pressure-resistance.
Therefore, the present invention seeks to provide a pipe in which the pipe wall is formed using thin metal belt plates in addition to a synthetic resin material or a rubber material. In some cases, the metal belt plates are exposed at the inner surface of the pipe wall. Despite the thin metal belt plates, the problem of flexibility is overcome. Additionally, the amount of the synthetic resin material required is greatly reduced so as to reduce the overall weight of the pipe, and the pipe is excellent in pressure-resistance.
According to the present invention, a pressure-resistant helically corrugated pipe comprises an integral, pipe wall corrugation helically wound to form a pipe, the corrugation having a trough portion disposed close to an axis of the pipe and outwardly projecting flange portions on opposite sides of the trough portion, adjacent flange portions being connected together to form the pipe having a helical outwardly projecting portion adjacent the trough portion. The pipe further includes a metal belt plate reinforcing member disposed within the trough portion of the helically disposed corrugation, the metal belt plate being generally U-shaped in cross-section so as to define a base, legs and free ends and opening toward the exterior of the pipe.
When the pipe of this construction is, for example, to be buried in the ground, a groove is dug in the ground at a required depth at the installation site, and the pipe is installed along the groove. When this pipe is used as a protective pipe for a cable, an electric cable or a telephone cable is passed through the pipe using a lead wire, and then the dug soil is applied to the pipe from above to cover it. In such installation of the pipe, when the pipe is required to be bent with a relatively small radius of curvature because of limitations imposed by geological features and geographical features of the installation site and existing building structures, the pipe of the present invention can be freely flexed to a certain extent since the connective belt region or regions 21 in which the metal belt plates are absent are provided either at the top portions 2 of the pipe wall 1 or at the top portions 2 and the bottom portions 5. Therefore, the pipe 1 can be easily installed without locally concentrating excessive tension and compression on the pipe 1.